This invention relates to flexible hydraulic hose and more particularly, to hydraulic hose that has very low volumetric expansion under pressure and which can withstand high pressure, maintain flexibility, resist kinking and is economical to produce.
Low volume expansion hose is necessary to obtain rapid hydraulic response from one point to another in a long line connection as for example in blowout preventer systems for offshore drilling rigs. In such blowout preventer systems, the hydraulic lines may be on the order of two thousand feet or longer and typically connect a control valve at one end to a pilot operated power valve at the other, operating at a working pressure typically on the order of 3,000 pounds per square inch. It is desired that response time be minimized in applications such as this. If conventional hydraulic hose were used in these applications, since it is made of materials to provide flexible characteristics, it would inherently have an accumulator like effect and a shock absorbing capability which reduces the response time of the system. These characteristics are desirable in many hose applications inasmuch as the expansion or bulging of the hose helps to dissipate pressure surges and the like and generally reduce shocks in the system. In a rapid response system, however, a reduced capability to expand is preferable for the hose construction, which stiff characteristics allow the rapid and precise transmission of pressure changes and the like from one end of the system to the other.
Currently, hose used for this purpose comprises a thermoplastic core tube, one or more reinforcement braids of aramid fiber and a thermoplastic jacket. In particular, a type 29 Kevlar fiber is employed which utilizes the extremely good tensile modulus characteristic of this aramid fiber in providing a strong and yet light weight hose construction which is suitable for extended lengths. While this construction is suitable for lines of moderate lengths, when longer lines are encountered, the time delay in transmission of pressure pulses throughout the system versus cost of the hose becomes more significant and improvement is desired in this regard.
A further problem with prior art hose constructions is the relatively high cost to produce very long continuous lengths of hose inasmuch as the aramid fiber employed is difficult to successfully braid without encountering excessive yarn breaks. In the past, it was necessary in braiding with the conventional aramid yarn, to pass the yarn through an expensive twisting operation and to braid with yarn having a twist of at least two turns per inch. Twisting of the yarn is recommended by the manufacturer to obtain maximum yarn tenacity and other satisfactory yarn characteristics. However, the modulus of aramid fibers is closer to that of fiberglass than to that of nylon or Dacron polyesters or the like, and this high modulus introduces significant problems in the handling of the yarns and a relatively high degree of filament or yarn breakage if satisfactory processing techniques are not employed. The high modulus of the armaid fiber is desirable in providing high strength capabilities within a small hose structure configuration and provides advantageous characteristics for the production of a low volumetric expansion hose, wherein, the integrity of the hose structure is desired to be retained.